Biden’s three rules for the war in Ukraine are working. He should stick to them.

The administration has executed on each rule. The Western financial and technological sanctions against Moscow are so comprehensive that Russian President Vladimir Putin claims they are “akin to an act of war.” The stream of U.S. weapons into Ukraine is helping to destroy hundreds of Russian warplanes and tanks, kill thousands of Russian soldiers and call into question Putin’s ability to achieve his war aims. The United States has also surged forces to fortify NATO’s position in Poland and in the Baltics.

But as the war continues and Ukrainian casualties mount — including from a weekend Russian strike deep into western Ukraine — calls for Biden to bend or break his rules are growing louder. Former director of national intelligence James R. Clapper Jr. suggested on Sunday that NATO would be forced into the conflict if “the Russians use chemical weapons in a siege of Kyiv.” Reporters are asking Western officials whether a Russian attack on weapons shipments in Ukraine would prompt military retaliation against Russia. Former State Department official Eliot Cohen writes in the Atlantic that NATO “could sweep the skies over Ukraine clear of Russian aircraft, and after a week or two of smashing Russian air defenses, devastate its ground forces.”

President Volodymyr Zelensky’s address to Congress on Wednesday is likely to put more pressure on Biden to change the terms of U.S. engagement in Ukraine. Americans want to support a valiant people’s defense of their sovereignty against a brutal invader.

Biden’s harrowing challenge in the coming weeks will be to resist this pressure. His three rules are not morally satisfying, but they are keeping the most dangerous military standoff between nuclear powers in decades contained. Altering those rules raises the risk that the destruction would spill out of Ukraine’s borders.

Consider: If the United States signals that it is prepared to attack Russia’s military directly in response to Russian actions in Ukraine, then what is the difference between Ukrainian territory and NATO territory? An advantage of Biden’s rules is that they delineate NATO’s perimeter as brightly as possible.

The bulk of weapons shipments to Ukraine are being transported covertly across Poland’s eastern border. But Putin knows, or should know, that any strike in Poland, a NATO member, would be met with a counterstrike an order of magnitude more powerful. That sets clear boundaries that can effectively deter Russia and contain the conflict.

If Russia comes to believe that strikes against weapons convoys in Ukraine will also prompt a direct Western retaliation, NATO deterrence becomes less meaningful. Russia might figure that once it weighs all the risks, a strike inside Poland is worth it if it can take out a larger number of Western weapons than a strike in Ukraine.

Biden can do more to enhance the United States’ deterrence. The president could add detail about what a Russian strike on even an “inch” into of NATO territory would mean — explaining, for example, that the Pentagon has dozens of Russian targets preselected which would promptly and unflinchingly be destroyed. He could send more missile batteries to NATO’s perimeter and increase assistance to groups fighting Russian-backed forces in the Mideast and elsewhere.

But it’s unlikely that bending Biden’s rules would limit Russian brutality in Ukraine. Western deterrence already failed to prevent Russia from invading a sovereign nation on multiple fronts with some 150,000 troops. After decades of give-and-take, NATO never mustered the will to admit Ukraine into the alliance, fearing precisely the war the world is now witnessing. The West faces a serious credibility challenge in reestablishing red lines in that country with an invasion already in process.

A new Biden rule — even one accidentally issued in a presidential gaffe — pledging U.S. military intervention inside Ukraine under certain circumstances would have to be enforced. Failure to do so would be catastrophic to U.S. credibility, far more than President Barack Obama’s unenforced red line in Syria against the use of chemical weapons. And unlike in Syria, enforcing such a red line would require direct U.S. attacks on a nuclear-armed military.

Russia was the original escalator of violence with its invasion, but U.S. intervention in the war theater itself would also be an escalation. Because of its inferior conventional forces, Moscow might have little room to escalate further besides deploying tactical nuclear weapons against NATO forces, knowing that its arsenal is larger and more flexible than that of the United States.

That could either force a humiliating U.S. climbdown or trigger an unpredictable escalatory spiral. Biden can best keep control of the conflict not by introducing new rules but by leaving no doubt through word and deed that the three already in effect — sanctions, weapons and the inviolable NATO perimeter — will be enforced to the hilt.